Charged particle beam microscopes, such as an electron microscope, are well known in the art, and are widely used during the manufacture of semiconductor wafers. For ease of discussion, the remaining disclosure makes reference to electrons as the charged particles; however, it should be appreciated that the discussion is equally applicable to other charged particles. The elements of a conventional electron microscope which are of particular relevance here are depicted in FIG. 1. Specifically, a vacuum chamber 10 houses an x-y stage 20 upon which the wafer 40 is placed by a robot (not shown). The chamber 10 is evacuated via outlet 70. The wafer 40 is introduced into the chamber 10 via a load lock 30 so as to avoid having to evacuate the chamber 10 each time a wafer is loaded
An electron column 50 is hermetically attached to the chamber 10. The column 50 houses the electron source and all the necessary electron optics (not shown). The column 50 is evacuated via outlet 60. The diameter of a conventional column is roughly 6–10 inches, while its height is roughly 15–30 inches. The conventional column is capable of providing an electron beam of sufficiently small diameter for wafer and reticle inspection, review and metrology.
One disadvantage of the prior art design is that whenever the column requires a repair which necessitates its removal from the chamber or breaking the vacuum in the column, the vacuum of the chamber is also broken. Breaking the vacuum in the chamber necessarily means that the microscope will be out of service for several hours. Another disadvantage is the requirement for separate vacuum systems for the column and the chamber, which increases the complexity and price of the system, while adversely affecting its reliability and stability.
Recently, a new type of column has been developed, and is generally referred to as a “minicolumn.” A cross section of a minicolumn investigated by the current inventors is depicted in FIG. 2. In FIG. 2, element 200 is the electron source (preferably a shottky emitter), 210 is an aperture (suppressor), and 220 generally designates the lens arrangement. More specifically, lens arrangement 220 comprises three lenses 230 made of conductive material and insulating spacers 240 interposed between the lenses 230. Ordered from the emitter, the lenses 230 comprise an extraction lens, a focusing lens, and an acceleration lens, respectively.
Notably, the diameter and height of such a column is measured in single-digit centimeters. More specifically, the diameter of the lens arrangement depicted in FIG. 2 is on the order of 3 centimeters, while its height is on the order of 1 centimeter. While this column is remarkably smaller than the conventional column, it provides an electron beam which has small diameter and was determined by the present inventors to be suitable for use in electron microscopes. Further information regarding the study of the minicolumn is presented in an article entitled “Novel high brightness miniature electron gun for high current e-beam applications” by F. Burstert, D. Winkler and B. Lischke, Microelectronic Engineering 31 (1996) pp. 95–100; and in an article entitled “Miniature electrostatic lens for generation of a low-voltage high current electron probe,” by C.-D. Bubeck, A. Fleischmann, G. Knell, R. Y. Lutsch, E. Plies and D. Winkler, Proceedings of the Charged Particle Optics Conference, Apr. 14–17, 1998.